GB2117193A - Electric discharge lamp operating circuit - Google Patents

Abstract

A series ballast inductor L and a power factor correcting capacitor C connected across the input terminals of the lamp operating circuit are coupled through a further inductance L1 connected to the ballast inductor L so as to adjust the resultant series inductance to a value suitable for a sodium lamp (SON), and an igniter circuit is coupled to the further inductance L1. The igniter circuit operates to apply ignition voltage to the sodium lamp (SON) during part of the time that the circuit is supplied with power. Protective circuitry C1 is coupled to the further inductance L1 to block or limit high frequency voltage pulses which could otherwise reach the ballast inductor L. The igniter circuit includes a thyristor Th1 and a capacitor C2 which is discharged resonantly into the lamp through part of the further inductor L1. A thermistor E restricts the current supplied to the igniter circuit so that during starting of lamp operation an internal starter of the lamp can ignite the lamp. <IMAGE>

Description

SPECIFICATION Electric discharge lamp operating circuit This invention relates to an electric discharge lamp operating circuit for an electric discharge lamp with an internal starter.

According to the present invention there is provided an electric discharge lamp operating circuit for a discharge lamp with an internal starter, the lamp operating circuit comprising a pair of supply input terminals, power factor correction means coupled to the input terminals, series inductance means connected between one of the input terminals and a first output terminal for connection to the lamp, and a combination of ignition circuitry and means for decreasingly restricting the current passed through the igniter circuitry, the said combination being coupled between the series inductance means and a second output terminal for connection to the lamp and being such that, in use during starting of lamp operation, voltages generated by the igniter circuitry allow the internal starter of the lamp to ignite the lamp.

The invention will now be described in more detail, solely by way of example, with reference to the accompanying drawings in which: Fig. lisa circuit diagram of an existing lamp operating circuit and a converter circuit.

Fig. 2 is a further diagram of the circuits of Fig.

1.

Figs. 3 and 4 are circuit diagrams of alterhative circuitry for parts of the converter circuit of Figs. 1 and 2.

Fig. 5 is a circuit diagram of part of an embodiment of the invention.

Fig. 6 is a detail diagram of a unit of the circuit of Fig. 5.

Fig. 7 is a circuit diagram of an alternative arrangement constituting a unit of the circuit of Fig. 5.

Fig. 1 shows an existing lamp operating circuit consisting of a series ballast inductor L and a power factor correcting capacitor C connected across the circuit input terminals, and a converter circuit to be added to the existing lamp circuit, which is intended to operate a high pressure mercury lamp (an MBF lamp), to produce a combined circuit capable of operating a high pressure sodium lamp (an SON lamp). The converter circuit includes a small inductor L1, which in use is connected in series with the existing ballast inductor L and corrects for the difference between the control current requirements of the MBF lamp and its replacement, the SON lamp. Most frequency, an existing MBF lamp should be replaced by a smaller SON lamp and therefore an increase in the series inductance in the circuit is usually required.

In addition hb the inductor LX, the converter circuit includes a small winding L;consisting of a few turns wound over the inductor L, so that the two inductors L, and L2 can act respectively as the secondary and primary windings of a step up transformer. An igniter circuit includes the winding L2, a resistor R1, a capacitor C2 and a thyristor Th, connected in series between the input terminals of the converter circuit. The trigger of the thyristor Th, is coupled by a triggering branch to an output terminal of the converter circuit at the output end of the inductor L,. The triggering branch consists of a zener diode Z, and a resistor R2 in series, the zener diode Z, being arranged to break down in response to positive voltage at the output end of the inductor L1.

When an a.c. mains power supply is initially connected to the input terminals of the existing lamp circuit with the converter circuit coupling it to the SON lamp, the SON lamp is open circuit.

Current flows down the triggering branch formed by the zener diode Z1 and the resistor R2 to the trigger of the thyristor Th, which fires shortly after the peak in each positive half cycle of a mains frequency voltage. When the thyristor Th1 fires, the capacitor C2 charges resonantly and the resultant current pulse in the winding L2 induces a pulse in the winding L, which results in a voltage pulse with a peak value of between 2 kilovolts and 5 kilovolts and a duration of between about 1 and 5 microseconds being applied to the lamp.

Such a voltage pulse ignites the SON lamp. After ignition the voltage across the SON lamp settles at a steady value lower than the ignition voltage, and too low for the thryistor Th, to be fired. The values of the resistors R, and R2 are chosen to ensure that substantially only the single high voltage ignition pulse is generated in each positive half-cycle of mains frequency.

The ballast inductor L is protected from the high voltage ignition pulses, and any other transient voltage pulses which the SON may generate in operation, by a buffer capacitor C1, which has a simple non-linear resistor R3 connected in parallel therewith, across the input terminals of the converter circuit.

The mercury lamp circuitry and the converter circuit of Fig. 1 are shown in Fig. 2 in which the converter circuit is represented as composed of three elements: the inductor L, which is connected in series with the ballast inductor L to increase the series inductance to a value suitable for a SON lamp; the igniter circuit indicated within a region A and providing high voltage for igniting the SON lamp; and the protective circuitry, represented by a block B, which may be a snubber circuit as in Fig. 1, consisting of the capacitor C, and the resistor R3, or another circuit capable of preventing transients and ignition voltage from reaching the ballast inductor L.

Fig. 3 shows elements A and L, again in which two diodes D, and D2 are included in the igniter circuit A. The diode D, is connected in parallel with the thyristor Th, but with reverse polarity. As a result, the capacitor C2 is charged to higher voltages than is the case in the circuit of Figs. 1 and 2. The diode D2 protects the trigger of the thyristor Th, from negative voltage.

Fig. 4 shows a further modification in which the elements L, and A are merged by providing the inductor L, in the form of a tapped inductor, the resistor R, being connected to the tap and there being no winding L2, the tapped inductor L, acting as an autotransformer.

Fig. 5 shows a converter circuit which converts an existing lamp operating circuit, not shown but consisting of a series ballast inductor and a power factor connecting capacitor connected across the operating circuit input terminals as in Fig. 1, into a circuit for operating a SON lamp fitted with an internal starter such as a bi-metal snap switch or a glow starter similar to the internal starter used in hot cathode fluorescent lamps. An example of such a SON lamp is a 50 watt or a 70 watt Phillips SON lamp with internal starter. SON lamps with bi-metal switches ignite almost instantaneously when the alternating supply is first switched on with the lamp cold. However, these lamps fail to re-strike when the lamp is hot, since the heat of the lamp causes the bi-metal switch to remain open for fifteen or more minutes after the lamp has extinguished.It is known to provide an external electronic igniter circuit for restriking such lamps, but the known igniter circuits have a tendency, especially where the internal starter is a glow starter, to interfere with the initial striking of the lamp, since the voltage generated by the external igniter circuit jumps across the internal igniter so that both the externally generated igniter voltage and the internally generated igniter voltage are short circuited and the lamp fails to strike.Use of a low voltage external igniter, i.e. 1 kilovolt instead of 2.3 kilovolts to 5 kilovolts, is unsatisfactory also in that the time taken to re-strike the lamp when hot is longer than for the higher igniter voltage and if the self-starting lamp is subsequently replaced with a lamp which does not have an internal starter, the replacement lamp is likely to fail to strike initially since 1 kilovolt is not sufficient to ignite most SON lamps when cold.

In the converter circuit of Fig. 5, the circuitry essentially corresponds to that of the converter circuit of Fig. 2 except for the additional provision of a unit E connected in series between the common rail 20 and the remainder of the converter circuit. A block capacitor C, is connected to prevent high frequency voltage from reaching the ballast inductor of the existing lamp operating circuit, this ballast inductor (not shown) being connected in series with the converter inductor L,.

Figs. 6 and 7 show alternative arrangements constituting the unit E of Fig. 5. In Fig. 6 the unit E is simply a thermistorT1. When a converted circuit in which the unit E is the thermistor T, is initially supplied with a.c. mains voltage, the thermistor T, is cold and is therefore in its high impedance state. The igniter circuit portion of the converter circuit operates as in Fig. 2 but does not generate significant voltage across the lamp, and the converter circuit does not place any significant impedance across the lamp.

Consequently the internal starter of the lamp is able to operate properly and ignite the lamp with the combined series inductance of the existing ballast inductance (not shown) and the converter inductor L, in circuit. As the igniter portion of the converter circuit continues to operate before the lamp strikes, current flows through the unit E, i.e.

the thermistor T1, so that the impedance of the unit E falls steadily. As a result, the voltage pulses generated across the lamp by the converter circuit increase in magnitude smoothly. It is found that the smoothly increasing externally generated voltage pulses assist the internally generated pulses and do not short circuit the contacts of the internal starter (not shown). The lamp is found to strike sooner than with the internal starter alone.

When the lamp strikes, the thyristor Th, ceases to be triggered and therefore the current through the unit E is substantially reduced and the impedance of the unt E becomes high again. If the lamp extinguishes while hot, voltage pulses are soon provided by the converter circuit which are high enough to effect rapid re-striking.

Furthermore, if the lamp is replaced with a SON lamp not having an internal starter, the voltage pulses generated by the converter circuit when the lamp is cold rapidly reach a magnitude sufficient to ignite the cold lamp. Thus the converter circuit monitors the lamp voltage and generates output voltages pulses whenever the lamp voltage is below the level predetermined by the triggering branch consisting of the resistor R2, diode D2 and zener diode Z, (which may alternatively be a Shockley diode). The output pulses increase smoothly in magnitude over a time determined by the unit E and reach a level at which the lamp strikes.

In Fig. 7, the unit E consists of a thyristor Th2 arranged to be controlled by a voltage divider formed by a series combination of an ordinary resistor R3 and a thermistor T, or some other nonlinear resistor whose impedance varies with current as in a thermistor. The thermistor T, is connected between the trigger and the anode of the thyristor Th, and the values of resistance presented by the resistor R3 and the thyristor Th2 are such that as the thermistor T, warms up and its resistance decreases, the voltage at the trigger of the thyristor Th2 reaches the level at which the thyristor Th2 fires. Hence, in operation with the lamp cold, the current allowed by the unit E of Fig.

7 starts from the small amplitude allowed by the series combination of the resistor R3 and the thermistor T, when cold, the thyristor Th2 being off, and increases smoothly as the thermistor T, warms up, until the thyristor Th2 fires whereupon the current increases substantially. The unit E of Fig. 7 conducts until the lamp voltage drops to the level at which the igniter circuit of Fig. 5 ceases operation, and then the thyristor Th2 turns off.

A practical circuit according to Figs. 5 and 6 for operating a 50 or 70 watt Phillips SON lamp with an internal starter has the following component values: Inductor L,: 300 turns tapped at 100 turns and wound on a 3/4 inch (1.9 cm) stack of No.

35 laminations from Linton and Hirst Ltd, England.

Thyristor Th1: TL 107 Capacitor C,: 0.02 microfarad Capacitor C2: 0.33 microfarad Zener diode Z,: PL 200Z Zener diode by SSC Ltd, England Diode D,: PY 127 diode by SSC Ltd Thermistor T,: VA 1056 S by Mullard Ltd, England Resistor R1: 3.3 kilohms.

The circuit of Fig. 5, with the unit E as in Fig. 6 or Fig. 7, can be combined with a further inductor and a power correction capacitor, connected to the input terminals of the circuit of Fig. 5 as the ballast inductor L and the capacitor C in the circuit of Fig. 2, to form an operating circuit for a sodium lamp with an internal starter where such a lamp is to be used as initio in a luminaire.Thus there is provided an electric discharge lamp operating circuit for a discharge lamp with an internal starter, the operating circuit comprising a pair of supply input terminals, power factor correction means coupled to the input terminals, series inductance means connected between one of the input terminals and first output terminal for connection to the lamp, and a combination of igniter circuitry and means for decreasingly restricting the current passed through the igniter circuitry during operation of the igniter circuitry, the said combination being coupled between the series inductance means and a second output terminal for connection to the lamp and being such that, in use during starting of lamp operation, voltages generated by the igniter circuitry allow the internal starter of the lamp ot ignite the lamp.

Matter described herein is also described and claimed in copending patent application No.

8030330 from which the present application is divided.

Claims (1)

1. Claims

   1. An electric discharge lamp operating circuit for a discharge lamp with an internal starter, the lamp operating circuit comprising a pair of supply input terminals, power factor correction means coupled to the input terminals, series inductance means connected between one of the input terminals and a first output terminal for connection to the lamp, and a combination of ignition circuitry and means for decreasingly restricting the current passed through the igniter circuitry, the said combination being coupled between the series inductance means and a second output terminal for connection to the lamp and being such that, in use during starting of lamp operation, voltages generated by the igniter circuitry allow the internal starter of the lamp to ignite the lamp.

   2. A lamp operating circuit according to claim 1, wherein the igniter circuitry comprises semiconductor switching means coupled to resonant circuit means whereby resonance voltages are established in the igniter circuitry by switching off the semiconductor switching means.

   3. A lamp circuit according to claim 1 or 2 wherein the series inductance means includes a tapped inductor forming part of the igniter circuitry.

   4. A lamp circuit according to claim 1, 2 or 3, wherein the semiconductor switching means comprises a thyristor having a trigger terminal coupled through a voltage breakdown device to an output end of the series inductance means.

   5. A lamp circuit according to any one of claims 1 to 4, wherein the said current means for restricting current through the igniter circuitry is connected in series with the igniter circuitry.

   6. A lamp circuit according to claim 5, wherein the means for restricting current through the igniter circuitry comprises a thermistor.

   New claims filed 18 May 1983 Superseded claims 1 New claim:

   1. An electric discharge lamp operating circuit for a discharge lamp with an internal starter, the lamp operating circuit comprising a pair of supply input terminals, power factor correction means coupled to the input terminals, and series inductance means connected between one of the input terminals and a first output terminal for connection to the lamp, a combination of igniter circuitry and means for decreasingly restricting the current passed through the igniter circuitry being coupled between part of the series inductance means and a second output terminal for connection to the lamp and being such that, in use during starting of lamp operation, voltages generated by the igniter circuitry allow the internal states of the lamp to ignite the lamp.